Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44


On Fri, 30 Sep 2016 15:35:26 -0300, you wrote:

2016-09-27 23:20 GMT-03:00 David @DWH [TekScopes] <

I will connect each diode and capacitor to High Voltage (that I will borrow
from my Kenwood
scope) and look for leakages, while heating them up with a heat gun...
If the problem is in those components, I will find.
That could work. I usually end up hacking together a quick high
voltage supply as needed. Periodically I consider designing a
variable high voltage low current bench supply for doing high voltage
leakage tests up to 3 kV or so.

So it helped but did not fixed the problem.
But what is the leakage at higher operating temperature? Whatever
part is causing the problem is only doing so after it warms up.
That's the difficult part... even with the original diodes,
which is the only thing I replaced so far, the problem takes quite
some time to happen with the outer case removed.
I am still puzzled as to why changing the diodes improved the
situation without fixing it. I have difficulty believing that the
diodes are leaking just enough to cause such a marginal problem.

If the diodes were the problem, then I would have expected changing
them to have completely fixed it instead of making it a little better.
I agree... I think that they're just helping to keep the whole thing cooler
and therefore, farther from heating up to the point of collapsing.
After doing a good HV test on them (as I mentioned above), I will put them
back... If they are not the culprits, it's nice to keep the scope original.
I usually do not care about keeping the stock condition if I can make
some improvements.

What I might try is looking for some other part which is getting hot
causing enough leakage to pull the high voltage supply voltages down.
If you do not have an infrared thermometer to scan with, a cotton swab
dipping in rubbing alcohol could be used to cool suspected parts after
the HV supply starts to collapse to see if it has any effect. Q1486
and Q1484 would be especially good targets for this test.
Hmmm... That is a good idea... I`m only afraid that a bare cotton swab
will not cool down Q1486 so easily!
So use a few of them at the same time. The heat of vaporization for
most liquids is really high which is why evaporative cooling works so
well. Water is especially good but common rubbing alcohol has quite a
bit of water in it.

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